1. Field of the Invention
The present invention relates to a molded-in transfer film, or a transfer film that is used in transferring pictures (or designs) to molded products simultaneously by injection molding, vacuum press forming or the like. The present invention also relates to a method for producing such molded-in transfer film.
2. Detailed Description of the Related Art
Transfer printing, commonly known as decalcomania, is a technique for providing pictures on molded products. In the transfer printing, a transfer layer carrying a picture or design is formed on a peelable substrate film and the resulting transfer film is used in such a way that the picture or design is transferred to another substrate for permanent fixing. A release layer may be provided, as required, on the peelable substrate film. In general transfer films, the transfer layer typically contains a strippable layer, a picture or design layer and an adhesive layer as essential parts. The adhesive layer is to insure that the transfer layer is positively transferred and bonded to the permanent substrate and may typically employ a heat-sensitive adhesive, a solvent-activatable adhesive or an ionizing radiation curable adhesive. The strippable layer protects the transfer layer after it is transferred to the permanent substrate. The release layer formed on the peelable substrate film facilitates the peeling of the transfer layer from the substrate film, and remains on the latter after transfer. The peeling occurs at the interface between the release layer and the strippable layer. The release layer is a film that is typically formed of a silicone resin, a melamine resin, etc. either by simple coating or other suitable techniques such as extrusion.
In recent years, proposals have been made that such transfer films be used to provide pictures on shaped parts simultaneously with their products as by injection molding or vacuum press forming. A brief description of the method for providing pictures simultaneously with injection molding is given below with reference to FIGS. 1 and 2 which should be looked at in combination with Examined Japanese Patent Publication Hei 4-42172 and Unexamined Japanese Patent Publication Hei 5-301250.
The apparatus 60 for implementing the method in FIGS. 1 and 2 generally includes a female mold 70 and a male mold 80 that is positioned side by side with respect to the female mold 70. The female mold 70 has a cavity 72 which corresponds to the outer profile of the shaped part to be produced. Aspiration holes 74 that are open to the cavity 72 are provided within the female mold 70, which is adapted to be driven by a drive unit 75 having a cylinder and the like so that it approaches or departs from the male mold 80. The male mold 80 has a core 82 that is to be fitted in the cavity 72; a gate 84 through which a molten resin is to be poured is provided within the male mold 80. If necessary, a heating platen 90 may be provided between the female mold 70 and the male mold 80 in such a way that it can be advanced or retracted as required.
To provide a picture or design simultaneously with injection molding on the apparatus 60, the following procedure may be taken. First, a transfer film 100 for providing the desired picture or design is placed on the lateral side of the female mold 70. If necessary, the film 100 may be heated to soften with the platen 90. Subsequently, the film 100 is held between the female mold 70 and the platen 90 to close the opening in the cavity 72. A vacuum is then drawn through the holes 74 in the female mold 70 while, at the same time, compressed air is supplied through air holes provided in the heating platen 90.
The transfer film 100 is then stretched along the inner periphery of the cavity 72 and brought into intimate contact therewith as shown in FIG. 1. The process up to this stage is generally referred to as "preliminary shaping or molding".
In the next step, the heating platen 90 is retracted and the female mold 70 is advanced as shown in FIG. 2, whereby it combines with the male mold 80 for mold clamping. Thereafter, the space of cavity formed between the two molds is filled with a molten resin that is supplied through the gate 84 in the male mold 80, thereby effecting injection molding.
As a result, the transfer film 100 in the female mold so becomes an integral part of the injected resin P and adheres to the latter so strongly that when the mold is opened after the end of the molding process, the shaped part having the transfer film 100 adhering to its outer surface can be recovered from within the mold.
In a subsequent step, the substrate film of the film 100 is peeled from the other parts of the film so that the transfer layer comprising the picture (or design) layer and other necessary layers is left intact on the shaped part S, whereby the desired picture (or design) is transferred to the latter.
As will be understood from the foregoing description, the success of the method for providing pictures simultaneously with injection molding requires that the transfer film 100 as heated to soften should be stretched sufficiently along the inner periphery of the cavity 72 so it has intimate contact with the latter (the need for good thermoformability) and that the substrate film of the transfer film 100 which has become an integral part of the outer surface of the shaped part S should positively be peeled from the other parts of the film (the need for good peelability). This is also true with other shaping methods, such as providing pictures simultaneously with vacuum press forming, that require the transfer film to be allowed to adhere, by a certain means, along the curved surface of the shaped part. The requirements are particularly critical when molding is done with a mold of great depth since the transfer film is subjected to deep drawing.
With a view to meeting these needs, many improvements in the transfer film have heretofore been proposed. They include, for example: a transfer film that comprises a heat-resistant biaxially oriented PET (polyethylene terephthalate) film as a substrate film which is overlaid with a release layer of a thermosetting resin such as a melamine rein that is baked and which, in turn, is overlaid by printing with a strippable layer and other necessary parts of the transfer layer (see Examined Japanese Patent Publication Hei 2-42080); a transfer film that comprises a thermoplastic and easy to mold PVC (polyvinyl chloride) resin as a substrate film which is coated with a release layer of a solvent-soluble polyamide which has good thermoformability an release property and which, in turn, is overlaid with a solvent-type strippable layer; and an improved version of the second type, which comprises a PVC film (substrate film) that contains 0 to 5 parts by weight of a plasticizer and which has good thermoformability, with an overlying release layer being formed by adhesive-bonding of a film of a polyamide resin such as nylon 6 or nylon 66 that are difficult to dissolve in organic solvents (see Examined Japanese Utility Model Publication Hei 4-5358).
The first-mentioned transfer film comprises the PET film that has a thermosetting resin such melamine resin baked at a temperature of ca. 80 to 150.degree. C. to form a release layer. which, in turn, is overlaid by printing with a strippable layer and other necessary parts of the transfer layer. This transfer film is capable of good release from an injected molten resin but due to the insufficient thermoformability of the PET film, it often fails to comply faithfully with the irregular shape of the mold. If the PET is replaced by a plasticized PVC film having good thermoformability, deformation will then occur on account of the heat of baking as supplied to form the release layer and, hence, it is difficult to manufacture practically feasible transfer films. It should also be noted that in either case, the release layer which is formed by baking a thermosetting resin such as melamine resin is so hard that when the transfer film is molded along the irregular shape, cracks are prone to develop in the release layer.
The second type of transfer film which has a PVC film coated with a polyamide release layer has good moldability; however, the solvent used in printing the strippable layer will dissolve the release layer which hence mixes with and adheres to the strippable layer and this occasionally causes difficulty in stripping the substrate film in a post-molding step.
The third type of transfer film has a PVC film overlaid with a release layer that is made from a slightly solvent-soluble nylon-base polyamide resin film and which is bonded to the PVC film with an adhesive. This transfer film exhibits a certain degree of moldability; however, the need to bond the release layer with an adhesive makes it inevitable to use a thick nylon-base polyamide resin film that should not be thinner than 10 .mu.m. Further, nylon-base polyamide resins themselves are insufficient in thermoformability to take advantage of the inherently good thermoformability of PVC. What is more, it is impossible to form solutions of conventional nylon-base polyamide resins and this is another factor that impairs the moldability of the transfer film. Polyamides that are soluble in organic solvents and which can be applied to form thin films are available by condensing dimer acids with ethylenediamine. However, the release layer that is formed of such solvent-soluble polyamides partly dissolve and swell when it is overlaid with the transfer layer by printing. Thus, it is unavoidable with the use of such special polyamide resins that the release layer mixes with and adheres to the transfer layer, making it difficult to peel the substrate film from the other parts of the transfer layer. As well as the problem of mixing and adhering, the use of PVC or other solvent-soluble resins in the substrate film not only softens but also weakens the latter upon printing the solvent-base release layer, whereby the printability of the substrate film is impaired.